LTE Interfaces

Explaining the Interfaces in LTE:

Within an LTE network, LTE Interfaces connect the various components to or within the core.

Interfaces allow the MME, SGW and PGW to cooperate with other network elements (e.g. HSS or PCRF).
Each one of them is built in a standard way described by 3GPP.org. Each interface described here is taken from the 23.401 3GPP.org documentation.

These LTE interfaces are used within either between eNodeBs and the core, or within the Evolved Packet Core (EPC)

CableFree LTE Evolved Packet Core

LTE Interfaces

The following are LTE Interfaces:

S1-MME :- Reference point for the control plane protocol between E-UTRAN and MME.

S1-U:- Reference point between E-UTRAN and Serving GW for the per bearer user plane tunnelling and inter eNodeB path switching during handover.

S4:- It provides related control and mobility support between GPRS Core and the 3GPP Anchor function of Serving GW. In addition, if Direct Tunnel is not established, it provides the user plane tunnelling.

S5:- It provides user plane tunnelling and tunnel management between Serving GW and PDN GW. It is used for Serving GW relocation due to UE mobility and if the Serving GW needs to connect to a non-collocated PDN GW for the required PDN connectivity.

S6a:- It enables transfer of subscription and authentication data for authenticating/authorizing user access to the evolved system (AAA interface) between MME and HSS.

Gx:- It provides transfer of (QoS) policy and charging rules from PCRF to Policy and Charging Enforcement Function (PCEF) in the PDN GW.

S8:- Inter-PLMN reference point providing user and control plane between the Serving GW in the VPLMN and the PDN GW in the HPLMN. S8 is the inter PLMN variant of S5.

S9:- It provides transfer of (QoS) policy and charging control information between the Home PCRF and the Visited PCRF in order to support local breakout function.

S10:- Reference point between MMEs for MME relocation and MME to MME information transfer.

S11:- Reference point between MME and Serving GW.

S12:- Reference point between UTRAN and Serving GW for user plane tunnelling when Direct Tunnel is established. It is based on the Iu-u/Gn-u reference point using the GTP-U protocol as defined between SGSN and UTRAN or respectively between SGSN and GGSN. Usage of S12 is an operator configuration option.

S13:- It enables UE identity check procedure between MME and EIR.

SGi:- It is the reference point between the PDN GW and the packet data network. Packet data network may be an operator external public or private packet data network or an intra operator packet data network, e.g. for provision of IMS services. This reference point corresponds to Gi for 3GPP accesses.

Rx:- The Rx reference point resides between the AF and the PCRF in the TS 23.203.

SBc:- Reference point between CBC and MME for warning message delivery and control functions.

More Information on LTE Interfaces

LTE Interfaces and their protocol stacks

Interfaces allow the LTE MME, SGW and PGW to cooperate with other network elements( e.g. HSS or PCRF).
Each one of them is built in standardised way described by 3GPP.org.

Each interface described here is taken from the 23.401 3GPP.org documentation.
Please keep in mind, that the documentation is (sometimes) bigger than we need it to be, so not every aspect of interfaces is described here.Let us start with a high level situation in which theuser is not roaming:

Fig.1. Non-roaming architecture by 3GPP

Figure above is showing only 4G interfaces, additional interfaces for 2G and 3G are described in TS 23.060.

The Serving Gateway (SGW) and PDN Gateway (PGW) could be put in one chassis.

As it comes to scenario with roaming architecture standards are describing two ways of dealing with it.

Two roaming scenarios:

where traffic is routed from home network to UE by S8 interface,

Fig. 2. Roaming architecture scenario with home routed traffic

where there is local breakout with home operator’s application functions only, and separate from that with visited operator’s application functions only.

Fig. 3. Roaming architecture for local breakout, with home operator’s application functions only

Fig. 4. Roaming architecture for local breakout, with home visitor’s application functions only

We should note that now we can go straight through interfaces their functions and protocol stack.

Information flow could be divided into two groups, one is Control Plane, and the other is User Plane.

The Control Plane consists of protocols for control and support of the user plane functions:

controlling the E-UTRA network access connections, such as attaching to and detaching from E-UTRAN;

controlling the attributes of an established network access connection, such as activation of an IP address;

controlling the routing path of an established network connection in order to support user mobility; and

S5: It provides user plane tunnelling and tunnel management between Serving GW and PDN GW. It is used for Serving GW relocation due to UE mobility and if the Serving GW needs to connect to a non-collocated PDN GW for the required PDN connectivity.

S8: Inter-PLMN reference point providing user and control plane between the Serving GW in the VPLMN (Visited PLMN) and the PDN GW in the HPLMN (Home PLMN). S8 is the inter PLMN variant of S5.

Difference between those two interfaces is S5 is used in one network entity(no roaming scenario), and S8 is being used to connect Visiting PLMN where user is with his Home PLMN.

Fig. 8. Protocol stack of interface S5 or S8

Where:

GTP-C: This protocol tunnels signalling messages between SGW and PGW.

UDP: This protocol transfers signalling messages between SGW and PGW. UDP is defined in RFC 768.

S10 interface between MME and other MME.
Reference point between MMEs for MME relocation (e.g handover) and MME to MME information transfer.

UDP: This protocol transfers signalling messages between MME and SGW. UDP is defined in RFC 768.

S6a interface between MME and HSS.

It enables transfer of subscription and authentication data for authenticating/authorizing user access to the evolved system (AAA interface) between MME and HSS.

Fig. 11. Protocol stack of S6a interface

Where:

Diameter: This protocol supports transferring of subscription and authentication data for authenticating/authorizing user access to the evolved system between MME and HSS (S6a). Diameter is defined in RFC 3588.

Reference point between CBC and MME for warning message delivery and control functions.

Cell Broadcast Center (CBC) was a solution for the special requirement of an Earthquake and Tsunami warning system (ETWS) created for Japan, introduced in Rel. 8. It utilizes the existing interfaces between UE and MME in control plane. Additionally the MME is connected to the CBC via the SBc interface. In LTE/4G SBc interface is fully standardized and based on SCTP.

It provides related control and mobility support between GPRS Core and the 3GPP Anchor function of Serving GW. In addition, if Direct Tunnel is not established, it provides the user plane tunnelling.

Fig. 16. Protocol stacks of S4 interfaces used to connect UE from 2G network to PDN

Where:

GTP U: This protocol tunnels user data between SGSN and the S GW as well as between the S GW and the P GW in the backbone network. GTP shall encapsulate all end user IP packets.

UDP/IP: These are the backbone network protocols used for routing user data and control signalling.

Protocols on the Um and the Gb interfaces are described in TS 23.060.

Fig. 17. Protocol stacks of S4 interfaces used to connect UE from 3G network to PDN

S12 interface between UE from 3G network and PGW.

Reference point between UTRAN and Serving GW for user plane tunnelling when Direct Tunnel is established. It is based on the Iu-u/Gn-u reference point using the GTP-U protocol as defined between SGSN and UTRAN or respectively between SGSN and GGSN. Usage of S12 is an operator configuration option.

Fig. 18. Protocol stack of S12 interface used to connect UE from 3G network to PDN

Where:

GTP U: This protocol tunnels user data between UTRAN and the S GW as well as between the S GW and the P GW in the backbone network. GTP shall encapsulate all end user IP packets

UDP/IP: These are the backbone network protocols used for routing user data and control signalling.

Protocols on the Uu interface are described in TS 23.060.

SGSN controls the user plane tunnel establishment and establish a Direct Tunnel between UTRAN and S GW as shown in Fig. 18.